1. Field of the Invention
The invention relates generally to flexible magnetic recording media and more particularly to flexible magnetic recording media having an improved porous undercoating film including particles of a non-magnetic oxide which imparts thick film properties to and increases the lubrication potential of the magnetic recording layer.
2. Description of the Prior Art
Magnetic media is commonly used to store and retrieve coded data. In the past, undercoating of the magnetic recording layer in the magnetic media has been carried out to coat over any non-uniformity of the base film substrate layer such as polyethylene tetrathalate. This is done to provide a good adhesion layer between the magnetic recording layer and the base film substrate layer, and to provide a conductive layer to reduce the potential for electrostatic problems. Today, the trend in magnetic media storage devices is toward higher bit densities to increase the data storage capacity. The most convenient way to achieve the higher bit densities is to increase the coercive force of the ferromagnetic iron oxide used in magnetic media. Since the magnetic field of the read-write head must saturate the higher coercive force magnetic media in order to properly switch the field, the thickness of the magnetic oxide must be reduced. Thus, magnetic recording layers of one micrometer thickness in flexible media and one-half micrometer thickness in rigid media are becoming common to solve this inherent problem.
However, thinner films have less durability and cohesive strength and are more prone to adhesion problems. The cause of lower durability is to some extent the influence of the substrate properties affecting the penetration hardness of the thin layer. The sudden change in elasticity of the materials used at the interface of the base film substrate layer and the magnetic recording layer reduces the durability of the recording layer. Thus, if a high durability substrate material is used, the magnetic layer will exhibit a higher level of durability than it otherwise would. Another major effect on durability of the thin magnetic recording layer is the quantity of lubricant that is available for lubricating the wear interface. These problems result in early magnetic recording layer failure.
Previous attempts to solve this durability problem which resulted in measurable increased wear resistance of the recording layer included the addition of hard additives to the magnetic oxide coating, the use of new materials to lubricate the recording layer, the choice of the particular magnetic oxide to be used to coat the recording layer, and a higher strength binder system that fuses the recording layer to the base film substrate layer. Additionally, in flexible media formulations, it is common practice to include lubricant in the magnetic coating formulation and/or to post coat the surfaces after coating. The lubricant is resident within the pores of the magnetic coating from which it is drawn to the surface as the surface lubricant is depleted during use. In particular, U.S. Pat. No. 3,959,553, issued to Hartmann, et al, discloses a process for manufacture of rigid magnetic disks which is especially suitable for manufacture of recording disks having very thin magnetic coatings. The magnetic disks can be manufactured very advantageously by coating a non-magnetic metallic base disk with a dispersion of a finely divided magnetic pigment in a solution of a binder in an organic solvent and converting the coating into a solid hard magnetic coating of predetermined hardness for recording if the non-magnetic metallic base disk, prior to application of the magnetic coating, is provided with a hard and polishable non-magnetic intermediate layer. This non-magnetic intermediate layer comprises a pulverulent mixture of finely divided inorganic non-magnetic pigments like .alpha.Fe.sub.2 O.sub.3 containing corundum powder having a particle size from one to twenty micrometers with a moh's hardness of at least seven or a finely divided cerium dioxide powder. Additionally, Hartmann discloses that acicular gamma-iron oxide may be employed as the magnetic pigments and that conventional small amounts of dispersing agents or lubricants may be used in the layers.
U.S. Pat. No. 3,996,095, issued to Ahn, et al, discloses a low temperature method of formation of magnetite Fe.sub.3 O.sub.4 and .gamma.Fe.sub.2 O.sub.3 films on a substrate which is not a single crystal. The results produced are useful as magnetic recording media and magnetic recording head layers because of the extremely smooth and stable iron oxide and ferrite thin films which display desirable magnetic characteristics. Additionally, the process can be performed successfully at a temperature of 200.degree. C. or less.
U.S. Pat. Nos. 4,075,384 and 4,259,392 issued to Suzuki, et al, and Suzuki, respectively, disclose inventions relating to multilayer magnetic recording media. In the '384 patent, Suzuki, et al discloses a magnetic recording tape comprising a non-magnetic support, a two-layered magnetic coating composed of a ferromagnetic powder and a binder formed on one surface of the support and in which the upper layer possesses a greater coercive force and dry thickness than the lower layer. Although multi-layered magnetic recording media have an actuating bias and equalization different from those of standard tapes and low noise tapes, Suzuki, et al discloses a multi-layered magnetic recording media with an actuating bias and equalization similar to those of a standard tape and a low noise tape and can be used in tape recorders not equipped with bias and equalization selectors. Also, the magnetic recording tape displays reduced head demagnetization and head wear. The '392 patent, discloses a multilayer magnetic recording media and dual-layer magnetic recording tape having improved magnetic recording properties and capable of providing the improved characteristics of a magnetic tape at a conventional bias and/or equalization position. Also, the dual-layer magnetic recording medium displays good linearity over broad frequency ranges and a broad dynamic range and good overall balance. The disclosure also provides for improved surface properties of a magnetic recording tape, the squareness ratio of the magnetic intensity (B) - flux density (H) characteristics and saturation property of the initial magnetization curve to redistribute the operation bias of the magnetic recording tape. Both Suzuki patents recite prior art disclosing magnetic coating composition and magnetic recording layers that may contain additives such as dispersants and lubricants.
U.S. Pat. No. 4,188,434 issued to Loran, discloses an invention relating to the use of solid and liquid lubricants. A magnetizable layer is fused to a substrate layer and then a solid and liquid lubricant layer is applied by dissolving the lubricants in a carrier solvent and then spraying the magnetic disk with the carrier. The solid lubricant is initially applied and buffed to a smooth layer and then the liquid lubricant is likewise applied.
U.S. Pat. 4,232,072 issued to Pardee, discloses an improved lubrication composition and method of its application to specified substrates. The improved wear-resistant, low-friction substrates have a coating provided by such composition and the substrates include sound and video recordings such as qramophone or phonographic records, video disks and the like. The composition is essentially comprised of carboxylate ester, tetrafluorethylene polymer, antistatic agent and a solvent.
U.S. Pat. No. 4,275,113, issued to Saito, et al, discloses a magnetic recording medium having a low surface resistivity of the magnetic layer with high quality electro-magnetic properties and adhesiveness between the magnetic layer and a carrier sheet. The structure enables one to avoid the problem of electrification of the recording medium and to maintain a good uniformity of the surface thereof, thereby enabling the suppression of electrostatic noise induced in the magnetic recording medium due to electrification. There is provided at least one intermediate layer with electrical conductivity between the magnetic layer and the carrier strip of the recording media. Carbon black, having a large specific surface area and a high oil absorption, is added as the conductive material in the intermediate layer. The particles of carbon black act as an electric conductive material to reduce the resistivity of the magnetic layer.
U.S. Pat. No. 4,321,303 issued to Morita, et al, discloses a magnetic powder for a magnetic recording medium having improved magnetic characteristics and obtained by forming a silica layer on the surface of an acicular iron oxide fine powder as a core by treating it with a silicon compound. The powder may be heat shrinked before preparing a cobalt modified acicular iron oxide fine powder. This occurs by dispensing the acicular iron oxide fine powder as the core in an aqueous solution of ammonia containing cobalt ions and treating it in an inert gas atmosphere at high temperature or in a reduction atmosphere. The acicular iron oxides used as the core can include hydrate type and can be .gamma.Fe.sub.2 O.sub.3 or .alpha.Fe.sub.2 O.sub.3. The result is a stable cobalt modified magnetic iron oxide.
Finally, U.S. Pat. No. 4,328,935 issued to Steel, discloses a magnetic recording tape having a non-magnetizable backside coating including smooth equant particles having a mohs hardness of at least seven. When the tape is used in a belt driven cartridge, the presence of the equant particles in the backside coating provides uniformity of tape speed. The equant particles of from 0.1 to 0.5 micrometers in average diameter are substantially free from sharp edges which reduces tape damage while on the reel.
In view of the prior art, the problem of flexible magnetic media durability still exists. With reference to the above recited prior art, some include an intermediate layer comprised of carbon, some include lubricants either in an overcoat of the magnetic medium or of the substrate, and some introduce hard ceramic particles as additives into the magnetic coating. However, none provide a satisfactory solution to the wear problem of thinner (1 micrometer) magnetic recording coatings.